This study compares the balloon air tonometry method of measuring gastric mucosal carbon dioxide (CO2) to standard saline tonometry. Also, it investigates the effect of histamine-2 receptor blockade on the precision of tonometric measures of gastric mucosal PCO2 (PgCO2). Measurements of PgCO2 were obtained from two gastric tonometers inserted orally in normal volunteers. One tonometer measured PgCO2 by the intermittent saline method, whereas the other measured PgCO2 using a newer continuous air method. Subjects received intravenous 5% dextrose during the first 6 hours of the experiment followed by a continuous infusion of a solution of ranitidine in 5% dextrose for another 6 hours. Measures of PgCO2 with the air tonometer method were similar to those obtained with saline tonometry. The reliability of PgCO2 measurements with either method improved with the use of ranitidine to maintain gastric fluid pH greater or equal to 5. This study is the first in a five part series. The second part of the series will utilize the air tonometer system to measure the effect of enteral feeding on PgCO2 in normal healthy volunteers. The third part of the series will repeat the assessment of the effect of enteral feeding, but will be conducted on a select group of critically ill patients. The fourth part will expand the third part to include a more diverse group of critically ill patients. Once the effect of enteral feeding on tonometered PgCO2 is established for normal and critically ill subjects, the fifth part of the series will attempt to utilize the measurement of PgCO2 to initiage early enteral feeding in the critically ill patient.